Starting times of the processes are $0,1,7$ and $5$ respectively.
Completion times of the processes are $17,5,10$ and $7$ respectively.
Turnaround times of the processes are $17-0 = 17, 5-1=4, 10-3=7$ and $7-4=3$ respectively.
So, average turnaround time $ = \dfrac{17+4+7+3}{4} = 7.75$